Characterization of nuclear spectroscopy tools mainly includes acquiring spectra in a variety of known test formations. For each elemental spectral response to be extracted from a spectrum, at least one specific test formation is currently used. Prior reference formations are primarily aimed at the characterization of neutron porosity, gamma-gamma density, and natural gamma ray measurements rather than the spectroscopy of neutron induced gamma-rays.
Conventional test formations used for the characterization of neutron porosity tools are cylinders approximately 5 ft in diameter and 5 ft in height, and are filled with a uniform distribution of test material. The dimensions of these formations are chosen to be large enough that any conceivable radiation detected by the tool is entirely contained within the test formation during its trip from source to detector. In combination with the uniformity of the test formations, the result is an environment which the tool interprets as being an infinitely large homogeneous formation typical of what might be encountered downhole. Formations of this large size can be used for the characterization of spectroscopy tools that detect neutron induced gamma-rays.
While convenient for typical measurements, the physical size of conventional test formations presents some difficulties. For example, the tank constructed to contain the formation is relatively expensive. The volume of material used to fill the formation is large, which limits the possibility of using costly, high-purity materials. Rather, quarried rock is generally used; locating rock with an appropriate composition and adequate purity is challenging and, in some cases, potentially impossible. Further, during tool characterization, many formations are transferred into and out of a small number of test pits. Large formations use heavy lifting equipment and pose safety hazards as they are moved around. Finally, storage is be provided for the many formations used in tool characterization, and the larger the dimensions of the formations are, the more difficult it is to find space for them.
The use of materials placed in proximity to a nuclear tool to influence its measurement, but which are not effectively infinite to the sensors in the tool, is also well known. The primary example is the use of inserts or sleeves during the calibration of nuclear tools. Materials of this type are used to calibrate both wireline and logging-while-drilling tools for density, neutron porosity, and neutron-gamma density measurements, for example. These materials can be engineered to achieve specific responses, see for example, U.S. Patent Application, Publication Number 2010/0180662, assigned to the same assignee as the present application, which is hereby incorporated by reference in its entirety.
Based on the foregoing, it would be useful in the industry to have a test formation apparatus which would address the size, cost and storage issues while still providing varied test formations for accurate elemental gamma-ray spectral responses.